The invention concerns a regulator for the electrical output of a generator, in particular wherein the generator comprises a Stirling engine (especially a linear free piston Stirling engine with linear alternator) and a method of regulating the electrical output of such a generator.
The application of a Free Piston Stirling Engine (FPSE) with a Linear Alternator (LA) to generate electricity in Domestic Combined Heat and Power (DCHP) units that provide hot water and central heating in a domestic environment is well known. Referring to FIG. 1, there is shown a typical power characteristic curve of a FPSE/LA, which is similar to that shown in U.S. Pat. No. 4,642,547. The power output of a FPSE/LA therefore depends on the head temperature. It may be understood that the amplitude and polarity of magnetic flux created by the permanent magnets changes when the power piston moves relative to the armature winding (stator). Power modulation methods which are drawn in FIG. 1 may include operating at a constant voltage or at a constant output power.
As explained in U.S. Pat. No. 4,873,826, the output power of a FPSE/LA is a function of the engine heat exchanger temperature ratio (in accordance with Carnot), operating frequency, mean pressure, and volumetric displacement of the displacer and the power piston. This document and U.S. Pat. No. 6,871,495 propose power modulation techniques based on mechanical controls. These present several drawbacks such as slow transient response, low reliability and higher cost.
However, power modulation can be achieved in the electrical side by means of regulating the load connected to the FPSE/LA which indirectly controls the current flowing through the alternator. The mechanical force acting on the piston (Fr) is proportional to the current flowing through the stator coil (i) and it provides an effective inertial load to the FPSE. This is shown with variation in time (t) in the equation below, where α is the LA motor constant.Fp(t)=α·i(t).
It is therefore desirable that such low-inertia generators can be provided with a suitable impedance across the generator terminals, irrespective of load demand. An impedance sensed by the alternator that is too high or too low could result in over-voltage, waveform distortion, and in extreme cases (such as an open or short circuit) physical damage of the generator engine.
The engine or alternator are normally assured of being presented with a reasonably stable impedance when connected directly to the electrical mains supply. However, there is often no inherent protection for the engine or alternator if they are used to provide electrical energy to connected appliances when disconnected from electrical mains, as in the case of a grid power blackout. A large number of techniques have been proposed for regulating the voltage in such scenarios.
As explained in U.S. Pat. No. 6,856,107, the induced voltage may be determined by assuming a sinusoidal flux waveform and, according to Faraday's law, its peak value is proportional to the amplitude of the power piston position. As can be seen from FIG. 1, the relationship between generated power and voltage seems to follow a quadratic relationship when the engine operates at a constant temperature gradient. Power modulation is performed modifying the operation point of the FPSE/LA, by controlling the head temperature and the load connected to the FPSE/LA.
Several power modulation control methods are known for keeping a constant displacer-to-piston stroke ratio and their relative phase angle by controlling the load connected to the FPSE/LA. A simple control strategy is described in U.S. Pat. No. 4,873,826, in which the winding ratio of an autotransformer is adjusted to maintain a constant voltage after the tuning capacitor. This strategy may be suitable for grid and off-grid applications. Another control method suitable for laboratory testing is based on a variable frequency power supply (inverter at a fixed output voltage), one autotransformer and a ballast load which is shown in U.S. Pat. No. 7,200,994.
Other approaches use an electronic load, which is a circuit that exploits the electrical characteristics of a power electronics topology in order to control the load impedance. Several known power electronic topologies may implement an electronic load. For example, U.S. Pat. No. 6,871,495 describes connecting different resistive loads to achieve voltage regulation in a DC bus, which is the rectified FPSE/LA output voltage after a tuning capacitor. To overcome the disadvantages of using a tuning capacitor to compensate the winding inductance of the alternator, an active rectifier has also been proposed in U.S. Pat. No. 6,856,107, U.S. Pat. No. 7,453,241 and U.S. Pat. No. 6,871,495. As suggested in U.S. Pat. No. 7,453,241 and U.S. Pat. No. 6,871,495, the active rectifier bridge transistors may be switched to control the phase of a SPWM (Sinusoidal Pulse Width Modulation) signal, until the alternator current is in phase with piston position or alternator EMF. The load is then connected in the DC bus and regulated using a voltage controller.
Several analogue and digital control techniques were proposed for the electronic loads to achieve power modulation of a Stirling engine. U.S. Pat. No. 7,453,241 proposes a strategy based on a constant voltage control in the DC bus side by means of using a hysteresis controller. US-2009/224738 and U.S. Pat. No. 6,871,495 consider digital control techniques using a reference sine wave in synchronism (or phase) with the FPSE/LA piston position or EMF.
Referring first to FIG. 2A, there is shown a first equivalent circuit for an existing regulator technology, which seeks to ensure a stable impedance in such cases using an electronic load. A generator comprising a FPSE/LA 10 has a tuning capacitance 12. The output voltage of the generator is measured by voltmeter 14 and an electronic load 22 is also across the generator output. The electronic load 22 is controlled by a voltage controller 20, which bases its control on the voltage measured by voltmeter 14.
However under such conditions, the load actually corresponds to the connected appliances that are connected across the electrical output of the alternator. These are termed customer loads and they may vary from zero up to the full rated output of the alternator. Desirably, the power of the engine should power up customer loads instead of damping the FPSE/LA generated power without any particular use.
Referring next to FIG. 2B, there is shown a second equivalent circuit for an existing regulator technology, which has similar components to those of FIG. 2A and these are identified by identical reference numerals. Customer load 30 is also across the generator output. When appliances are first connected to the generator these loads may demand “inrush” currents that are greatly in excess of those normally provided by the alternator. Inductive loads could also require high levels of current for short periods of time. Therefore, an adequate regulation strategy is demanded to ensure that such a low inertia generator is presented with stable impedance under all load demand conditions. Therefore, typical power modulation techniques are based on controlling the impedance connected to the FPSE/LA as it can provide control of the displacer to piston stroke ratio and their relative phase angle.
Referring now to FIG. 2C, there is shown a third equivalent circuit for an existing regulator technology, which has similar components to those of FIGS. 2A and 2B and these are identified by identical reference numerals. In addition to the voltage control 20, a power control block 40 is also placed at the generator output. An equivalent circuit for a regulator technology in accordance with US-2009/189589 (commonly assigned with this invention) is shown in FIG. 2D. Again, where the same components as those of FIG. 2C are used, they are identified by identical reference numerals. The power control block 40 of FIG. 2C comprises a voltmeter 46 and a current meter 48, which provide measurements to power control block 42. This controls an AC chopper 44 to affect the output voltage across the customer loads 30.
Determining the control signals for the voltage control block 20 and power control block 42 is not straightforward. US-2009/189589 suggests a technique for determining an error signal that can be used to modulate the AC input signal in order to obtain a regulated AC output signal. Referring now to FIG. 3A, there is shown a schematic diagram of a method for regulating the AC signal, as described in this document. The AC input signal is sampled to produce a sampled AC signal as shown by input waveform 52. This is provided to full-wave rectifier 56 and the rectified AC signal 58 results. In parallel, the sampled AC signal 52 may be used to generate trigger pulses 62 to coincide with the sampled AC signal 52 crossing through zero volts. This zero crossing may be detected using software and may use digital filtering to remove the effects of noise around the zero crossing and make use of software pattern matching to improve face synchronisation. A computer 64 uses these trigger pulses 62 to generate a synchronised reference signal 66. The reference signal 66 corresponds to a sinusoid, but with only positively extending lobes such that it is equivalent to a full-wave rectified AC signal.
In ratio-metric comparison block 70, the scaled AC signal 58 is subtracted from the reference signal 66 to produce an error signal 72. In other words, instantaneous values are subtracted from instantaneous values. To ensure that only positive values are obtained, an offset is introduced. For example, this subtraction may be implemented in a difference amplifier operating with a suitable offset.
This error signal 72 is not only a function of the amplitude difference between the scaled AC signal 58 and the reference signal 66, it is also a function of the phase of the AC input signal. This phase variation may be removed by a multiplier chip 76 that operates to divide the error signal by the reference signal 66 to provide a percentage error signal 78. This percentage error signal 78 may then be used to modulate the AC input signal.
This is a relatively fast-response control loop, as instantaneous changes in the error signal are immediately reflected in changes to the modulation. Referring now to FIG. 3B, there is shown a further schematic diagram, illustrating the generation of the exact error signal for controlling the modulation. The AC input voltage 52 and the design output voltage 66 are provided to the ratio-metric comparison block 70, which generates the percentage error signal 72. This is passed to a proportional controller 76 which generates the analogue error signal 78. This is compared with the output of a ramp generator 77 using a comparator 79 to generate a Pulse Width Modulation (PWM) signal 81, which is fed to an electronic load such as an AC/AC Buck regulator (not shown), also called an AC/AC chopper.
This control strategy and error definition effect power control (that is, regulation of the voltage when it begins to drop, indicating that a higher than normal load is being applied), because the error signal is positive when the generator output (as scaled) is less than the reference signal. This may deal with in-rush currents, as explained above. The error definition changes when voltage control is required (that is, regulation of the voltage when the load applied is within a normal range), such that the error signal is positive when the generator output (as scaled) is greater than the reference signal.
By generating an error signal that is expressed as a fraction (that is, a percentage) of the reference signal, the error signal magnitude is essentially independent from the LFPSE/LA AC voltage level. This makes it suitable for use with a fast proportional controller, such as proportional controller 76. This complex control strategy appears to be well-suited to generators based on low inertia engines, for example, Stirling engines, in which a regulation strategy that minimises the risk of mechanical failure is demanded. However, it requires a significant amount of regulation-specific processing.